Best place to find one is a salvage yard because they have a very low failure rate so your chance of getting a bad one is low. What you must do though is find one from a car with the same options as yours because they're calibrated according to the weight distribution of the vehicle. If you can find a part number on them that would be even better. Be aware too that the part number on the assembly is often different than what the dealer goes by. They use the vehicle manufacturer's part number. The part can have the part number of the supplier of that part which will be different.

Nothing really to find out. I just wanted to point out that if you already checked at the dealership and they gave you the part number, there's a real good possibility you could find a different number stamped or printed on the part. If you find a valve in the salvage yard with the same number(s) as on your old one, they should be the same.

Some of the things that can make a difference is with or without air conditioning, manual or automatic transmission, two or four door, 4 cylinder or V-6 engine, and anything else like that which affects the percentage of weight on the front tires. The valves will also be different between rear drum and disc brakes.

The goal is to find a car equipped identically to yours so it has the same weight distribution. The proportioning valve is calibrated to reduce rear wheel lockup under moderate to hard braking when more weight shifts to the front.